The electrical circuits on both land and aeronautical vehicles commonly include a battery serving as a current storage reservoir and an alternator serving as the current source, with solid state diodes serving to rectify the alternating current produced by the alternator to a direct current.
For most of the applications needing electrical power on an aircraft, such as lights, landing gear actuators, and hydraulic power sources, a small amount of an AC component of the DC power is not objectionable, the normal output of the alternator system including less than 1% of ripple current.
An alternator can fail in several ways. A short circuit can develop in one of the windings, causing excessive current to flow through the shorted winding due to the drop in resistance, lessening the amount of magnetic flux due to the loss of the shorted windings from the circuit. This in turn will lessen the total output of the generator and set up an imbalance in the amount of current produced by the windings, causing an AC ripple in the DC output of 5% or more. A failure of one of the rectifying diodes will also cause an imbalance, thereby increasing the AC component of the total current.
The equipment is generally designed with generous overload tolerances, however, and failure of one part does not usually cause immediate failure of the whole system. The alternator will continue to function, but the increased electrical stress placed upon the system will eventually cause an increase in physical stress, usually in the form of heat, or arcing, and the total system will fail in an avalanche fashion. The increased AC component of the current may also cause some equipment malfunctions in those systems particularly sensitive to current variations such as the instruments and radio equipment. Any malfunctions in these areas are of particular concern to the safety of aircraft for obvious reasons.
The period of time from the appearance of the first symptoms of incipient failure until the occurrence of complete failure may vary from several minutes to many hours of operation in typical aircraft systems. This interval has heretofore been one principally of blissful but deadly ignorance on the part of light plane pilots in particular. This invention provides for the first time a simple, light, and economical warning of such an incipient failure.
Previous efforts in the area include the following known U.S. patents and other references:
U.S. Pat. No. 3,168,693 Eckenfelder Feb. 2, 1965, discloses a voltage regulator which uses the DC ripple to determine the total voltage output, and to regulate the exciter circuit of AC generator. This device will limit the exciter current and presumably prevent further damage in case of a component failure but will not give warning to the pilot unless he is watching the generator output meter. PA1 U.S. Pat. No. 3,210,603, Calfee, et al., Oct. 5, 1965, discloses a system to protect brushless AC generators from rectifier failure or exciter armature winding insulation failure. The ripple current and the ripple frequency of the exciter field, at a predetermined level, activate a Zener diode to interrupt the exciter field current. This system, like Eckenfelder above, will limit exciter current but will not give any warning to the pilot. PA1 U.S. Pat. No. 3,479,575, Wright, et al., Nov. 18, 1969, discloses an indicating device for a battery charging current. This system will indicate only a total failure of the electrical generation system and will not detect an incipient failure. PA1 U.S. Pat. No. 3,492,559, Harris, Jan. 27, 1970, discloses a voltage sensitive indicator which activates a warning light when the voltage is too high or too low. This device will only, like Wright above indicate when generator output is grossly in error, and will not detect an incipient failure.
Further background is shown in the brochure copy enclosed from Ward Aero, WA 165, Rev. 2-75. No information is available on the circuitry involved in this instrument.